1. Field of the Invention
The invention relates to a focussing and focal length adjusting device for a video camera, in particular for an optical lens system in an endoscopic video camera, with a drive device moved by at least one piezoelectric actuator, for a focussing lens of the lens system, with an automatic focussing circuit for producing a focus adjusting signal which can be supplied to the piezo electric actuator, and with an electromotoric zoom drive, connected to a zoom objective of the optical lens system, with a zoom control circuit which with a manually inputted zoom signal produces a focal length adjusting signal for the zoom drive.
2. Discussion of the Prior Art
Particularly with endoscopic applications with which the operating surgeon does away with an assistant guiding the camera and holds the camera himself or has it fixed by way of a mechanical holding arm, the automation of the frequent picture focussing readjustment means a significant handling improvement which permits the operating surgeon to concentrate more on the surgical or diagnostic activity. In endoscopy an autofocus may only be accepted when it reacts unnoticeable to the human eye, from which there results the demand that the used control system as well as the drive must be very quick and that such a drive, neither in the camera head nor in the cable to the camera controller, may cause disturbances of the picture signal.
Autofocussing devices in conventional video cameras are frequently realized by electric motors which drive a linearly guided focussing member via a spindle. A further usual movement transfer of electric motors is effected via a toothing on the circumference of a sleeve containing the focussing member, in order to rotorically move this. This rotational movement by way of a helical slot in the sleeve is converted into a translatory adjustment of the focussing sleeve. The use of electric motors in endoscopic camera heads often entails disturbances in the video picture which by way of the nature of the use may not be accepted. Furthermore electric motors may only be moved successively in one direction, and a jump-like movement to a certain target position is not possible with them. Thus with electric motors the demands of a rapid focus adjustment unnoticeable to the eye can only be fulfilled with difficulty.
From U.S. Pat. No. 5,490,015 there is known an automatic focussing device for a video endoscope which comprises a piezoelectric actuator for moving a focussing lens. This known focussing drive is based on the properties of a piezoelectric stack actuator, with which several platelet-shaped piezoelectric ceramic elements and intermediately lying electrode layers are stacked over one another and which extends jump-like with a suitable activation. The piezoelectric actuator positioned in a groove by a clamping device overcomes the sticking friction by way of a mechanical impulse which results from a jump-like extension. By way of this the actuator may slide into the groove and the focussing lens holder connected to the actuator may be moved over a short period of time. The successive repetition of this short movement section leads to a perceivable adjustment. The direction forwards or backwards with this is determined by the shape of the flanks of the activation voltage which is applied to the piezoelectric actuator. In order however to achieve an adequate adjusting path the piezoelectric actuator receives a sequence of voltage impulses within a certain space in time. These voltage impulses may in the video picture lead, to visible disturbances since a certain cross talking onto the video signal leads in the camera cable cannot be avoided. The same applies to the direct neighborhood of the control actuators to the sensitive camera head.
In the field of vario-objective control, i.e. for activation of a zoom objective of a camera there are known a series of drive systems. From U.S. Pat. No. 5,349,408 and U.S. Pat. No. 5,652,922 there are known motoric drive units for vario-objectives which use a synchronous motor or bush motor and for transmission use pinions, gearwheels and helix guides. For the application in standard video cameras this is a practical solution. In the field of endoscopic video cameras however by way of the operation of such electric motors disturbances in the video picture arise. Furthermore these electric motors are not suitable as drive units in video endoscopes on account of their mostly considerable space requirement.